A log raft is 6-10 logs of similar diameter, 20-30 cm across and 3-4 m long, laid parallel with the largest on the outside, lashed to at least two cross-poles, one at each end, with a raised platform of planks above the logs and a sweep oar at the stern. Eight seasoned softwood logs 25 cm by 4 m weigh about 707 kg themselves and, at 450 kg/m³, carry 277 kg of cargo with 10 cm of log freeboard or 468 kg with 7.5 cm, short of the 400-500 kg that “half of 800-1,000 kg” would give; the overview’s figure needs a wood of only 255-318 kg/m³, and green logs at 800 kg/m³ cannot carry their own weight at those freeboards.
Part of Coracle & Small Watercraft
A log raft is a floating platform of lashed logs for river crossings and cargo, with a capacity that follows directly from the logs’ size and density.
Capacity Is A Question Of How Deep The Logs Sit, And In What Wood
The carrier’s design: select 6-10 logs of similar diameter (20-30 cm), 3-4 m long; lay them parallel with the largest logs on the outside; lash them with cross-poles and rope (drilling holes and threading rope through gives the strongest connection; at minimum use two cross-poles, one at each end, lashed to every log); build a raised platform of planks to keep cargo above water that washes over the logs. For steering, a long sweep oar (a pole 5-6 m with a blade lashed to the end) at the stern; on rivers, a pole for pushing off the bottom in shallow sections. The carrier states a capacity: an 8-log raft of 25 cm × 4 m floats approximately 800-1,000 kg including its own weight, half of which is useful cargo.
The arithmetic tests that. The volume of 8 logs is 8 × π × 0.125² × 4 = 1.571 m³. The mass of the wood is 1.571 × ρ. The displacement when the logs are submerged to depth h follows the circle-segment formula: area = r² arccos((r − h) ÷ r) − (r − h) √(2rh − h²), times length times 8 times 1,000 kg/m³.
What You Need
- 8 logs, dry, straight and of similar diameter, each float-tested
- Cross-poles and rope for lashing; drill holes or notch the logs so that rope cannot slide (see Lashing Techniques)
- Planks for the platform, laid on beams above the logs
- A sweep oar and poles
- A scale or a way to estimate the load
Method: Size, Lash, Test
Step 1: Float-test each log: weigh a short length or note how deep it sits.
Step 2: Compute capacity for the wood density you have, for several immersion depths, and pick a depth that gives the freeboard you want.
Step 3: Lash the logs to two or more cross-poles, with each log lashed at every cross-pole.
Step 4: Mark the waterline that corresponds to your chosen load.
Step 5: Load in steps, and stop at the mark.
Worked Example: Eight 25 cm Logs
Total log volume 1.571 m³. Own mass: 628 kg at 400 kg/m³, 707 kg at 450, 785 kg at 500, 1,257 kg at 800. Displacement (kg) and log freeboard by immersion depth h as a fraction of the diameter:
| h ÷ d | Displacement | Log freeboard | Cargo, 450 kg/m³ | Cargo, 400 | Cargo, 500 | Cargo, 800 |
|---|---|---|---|---|---|---|
| 0.5 | 785 kg | 12.5 cm | 79 kg | 157 kg | 0 kg | none |
| 0.6 | 984 kg | 10.0 cm | 277 kg | 356 kg | 199 kg | none |
| 0.7 | 1,174 kg | 7.5 cm | 468 kg | 546 kg | 389 kg | none |
| 0.8 | 1,347 kg | 5.0 cm | 640 kg | 719 kg | 562 kg | 90 kg |
(Cargo = displacement − log mass; the platform and the lashings, assumed at a few tens of kilograms, come out of it.) The carrier’s total of 800-1,000 kg is 51-64% of the 1,571 kg fully-submerged displacement. For half of it to be cargo, the logs would have to weigh 400-500 kg: a density of 400-500 ÷ 1.571 = 255-318 kg/m³, lighter than the 400-500 kg/m³ range for seasoned softwood in Logistics & Load Planning. At 450 kg/m³ a realistic raft carries about 277-468 kg at 10-7.5 cm of freeboard. Green logs of 800 kg/m³ outweigh the displacement at the first three depths in the table, and at 0.8 they leave only 1,347 − 1,257 = 90 kg of cargo with 5 cm of freeboard; fully submerged the raft displaces 1,571 kg against 1,257 kg of wood, a cargo of at most 314 kg with the deck awash.
Stability: at 7.5 cm of freeboard the deck-edge angle for a 2.0 m wide raft is arctan(0.075 ÷ 1.0) = 4.3°. Treating the waterplane as a 4 m × 2 m rectangle, BM = I ÷ V = (4 × 2³ ÷ 12) ÷ 1.174 = 2.3 m, very stiff at small angles, but the logs are awash at about 4° of heel and the cargo gets wet or slides. Stiffness at small angles is not the same as range.
Common Mistakes
- Counting the carrier’s “half” as cargo — it depends on the wood density.
- Unseasoned logs — they float so low that there is no payload.
- Lashings only at the ends of the logs — they work loose; use at least two cross-poles and lash every log to each.
- No platform — cargo placed on round logs rolls.
Safety
The raft that feels stable has almost no freeboard
At 7.5 cm of log freeboard, a 5 cm wave or a few degrees of heel puts water over the deck, and logs can roll apart if the lashings fail. Mark a load line, tie the load down, wear flotation, and keep off fast water. If the raft breaks up in cold water, hold a log and follow Safety & Capsizing Recovery.
What’s Next
- Learn next: Barrel & Drum Raft — more lift for a given size
- Combine with: Ferry Platform Design — when a raft becomes a ferry